1. Field of the Invention
The present invention relates to sintered Bi.sub.2 Te.sub.3 -based thermoelectric materials containing Ag.sub.2 S which prevent p- to n-type transition of Bi.sub.2 Te.sub.3 -based thermoelectric materials during polycrystalline sintering and have improved thermoelectric properties.
2. Description of the Prior Art
Thermoelectric materials based on Bi.sub.2 Te.sub.3 have high figures-of-merit (Z) at room temperature (Z.about.1.5.times.10.sup.-3 K) and, thus, have been drawn much attention as one of the most promising thermoelectric materials for low temperature applications. The materials are usually employed in the form of a single crystal for thermoelectric devices. Single crystalline Bi.sub.2 Te.sub.3, however, is extremely weak mechanically due to easy cleavage (P. D. Delavignette, "Dislocation nets in Bi.sub.2 Te.sub.3 and Sb.sub.2 Te.sub.3 " Phil. Mag., 5, 729-744 (1960)) and is normally subject to difficulties with mass production. In order to circumvent these drawbacks associated with the Bi.sub.2 Te.sub.3 -based single crystalline material, attempts have been made to prepare polycrystalline Bi.sub.2 Te.sub.3 via sintering. However, polycrystalline Bi.sub.2 Te.sub.3 -based materials go through p- to n-type transition during sintering (W. R. George, "The Sintering of Bi.sub.2 Te.sub.3 " Proc. Phys. Soc., 74, 768-770 (1959)). Such a change of electrical condition behavior from a p-type (holes) to an n-type (electrons) during processing causes thermoelectric power and conductivity of the material to be decreased. It is believed that p- to n-type transition occurs due to a defect of donors in sintering. One always ends up with n-type material after sintering even if started with p-type powder, which have prevented p-type thermoelectric material based on Bi.sub.2 Te.sub.3 from being fabricated via the powder metallurgical technique. It is thus necessary to prevent, while maintaining the high figure-of-merit, the p-n transition of the Bi.sub.2 Te.sub.3 -based thermoelectric materials in sintering.